Field of the Invention
Various embodiments of the invention relate to the area of optical communications photonic integrated circuits (PICs). In particular, various embodiments relate to integrated optical transmitters capable of generating multilevel optical modulation formats in optical communications applications.
Description of the Related Art
As demand for higher capacity in optical networks continues to grow, ways to increase optical network capacity with reduced capital investment are of interest. One cost efficient solution that allows for increased or maximum utilization of the existing optical network infrastructure is to implement more spectrally efficient modulation formats for increased data throughput capacity. Advanced modulation formats such as Quadrature Amplitude Modulation (QAM), Phase Shift Keying (PSK), and Quadrature Phase Shift Keying (QPSK) are spectrally efficient and can improve the efficiency of fiber Wavelength Division Multiplexing (WDM). Modulation formats such as Quadrature Phase Shift Keying and Quadrature Amplitude Modulation can allow for a number of data symbols to be sent utilizing the same line rate as a lower bit-rate On-Off keyed system. Present optical transmitters for generating optical signals having advanced modulation formats are large-scale monolithic photonic integrated circuits (PICs) or use hybrid platforms. Due to their size, large-scale PICs may require low waveguide losses, high performance optical sources and other optical subcomponents. Moreover, refined fabrication processes and techniques may be required to reduce defects and to improve yield of large-scale PICs. Thus, there is a need for optical transmitters having a reduced footprint and a tolerance for optical waveguide losses that can be fabricated with simple integration platforms.